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Target Outcome

Measurements of six degree of freedom kinetics and kinematics of the tibiofemoral joint under quasi-static loading conditions
Measurements of six degree of freedom kinetics and kinematics and contact pressures of the patellofemoral joint under quasi-static loading conditions

Prerequisites

Infrastructure

For more details see ["Infrastructure/ExperimentationMechanics"].

Prerequisite Protocols

["Specifications/Specimens"].
["Specifications/SpecimenPreparation"]
["Specifications/Registration"]
["Specifications/ExperimentationAnatomicalImaging"]
["Specifications/PressureCalibration"] (for patellofemoral joint testing)

Protocols

Robot-Specimen Assembly & Alignment

The testing procedures require placement of the specimen on the robot and following the necessary set-up procedures. These are detailed in the attachment:simVITRO_setup_tutorial.pdf

Tibiofemoral Joint Testing

Primary Conditions

Joint laxity at 0°, 30°, 60°, and 90° flexion angles for three isolated degrees of freedom:
1. internal-external rotation,
2. varus-valgus, and
3. anterior-posterior translation.
Combined loading; permutations of internal-external rotation moments, varus-valgus moments, and anterior-posterior drawer forces.

Secondary Conditions

Joint stretching at various flexion angles, to potentially identify gross mechanical properties of the ligaments

Measurements

Joint kinetics
- Joint kinetics are calculated based on robot position and 6-axis load cell output.
- reaction forces (anterior-posterior, compression-distraction, medial-lateral)
- reaction moments (flexion-extension, internal-external rotation, varus-valgus)
- provided in anatomical joint coordinate system
- provided in a human-readable or openly accessible form with clear descriptions of data structures and file naming/storage conventions
Joint kinematics
- Joint kinematics are calculated in two different ways:
  - Based on Optotrak sensor output
  - Based on robot position output
- translations (anterior-posterior, compression-distraction, medial-lateral)
- rotations (flexion-extension, internal-external rotation, varus-valgus)
- provided in anatomical joint coordinate system
- provided in a human-readable or openly accessible form with clear descriptions of data structures and file naming/storage conventions

Equipment Use Modes

Refer to ["Infrastructure/ExperimentationMechanics"] for details of the equipment utilized in this specification.
Tibiofemoral joint testing will be done via the six degrees of freedom robot (Rotopod R2000, PRSCO, Hampton, NH) with a rotary stage mounted to the top to yield a seventh degree of freedom for increased range of motion.
An Optotrak motion tracking system (NDI, Ontario, Canada) will used to digitize the robotic system hardware and specimen anatomy, to create and control motions and loads in an anatomical knee joint coordinate system specific to the specimen (Grood and Suntay, 1983).
A universal force sensor (UFS) will be attached to the robot platform, or may be attached to the frame surrounding the robot depending on the range of motion considerations of the particular loading condition. The tibia will be fixed to the UFS.
The robot will be operated in real-time force feedback control so that loading trajectories can be applied to determine the five degrees of freedom kinematics of the joint. Flexion will be prescribed, as in position control.

Operating Procedure

Coordinate System Optimization:

A kinetics based neutral position (minimal loading) will be established once the femur is set to 0° flexion angle by operating the robot in force control mode.
The tibiofemoral joint will be moved under a passive flexion from 0° to 90° flexion with a 100 N compressive load. The femur coordinate system will then be optimized to minimize the change in joint translations and off-axis rotations throughout the cycle. This process is intended to objectively define a functional mechanical axis of the femur. Some limitations with this approach may exist in that the screw home mechanism, i.e. off-axis rotations, may be minimized.
The kinetics based neutral position will re-established with the refined coordinate system.

Preconditioning:

The specimen will be preconditioned by manually loading the knee and by applying the terminal loads of the laxity loading protocols once for 30° flexion.

Testing of Primary Conditions:

Testing protocols were adapted from Borotikar (2009).
The data will be collected and stored based on filing naming and data storage conventions utilized by the BioRobotics Core of the Cleveland Clinic, see ["Infrastructure/ExperimentationMechanics"].
Throughout all tests, a 20N axial compression will be included to maintain tibiofemoral joint contact.
The actions during the timeline of testing will be:
1. Apply AP laxity loading at 30° flexion:
  - Anterior/posterior force of 0 to ±100 N in steps of 10 N.
  - Note that this loading condition will be repeated throughout testing to ensure there was no injury or damage to any of the structures in the joint. It will also quantify small changes in joint laxity that may occur during testing due to repetitive loading.
2. Set flexion angle to 0°.
3. Apply laxity loading:
  1. Internal-external rotation: 0 to ±5 Nm in steps of 1 Nm.
  2. Varus-valgus: 0 to ±10 Nm in steps of 2.5 Nm. 0 to ±100 N in steps of 10 N.
  3. Anterior-posterior translation: 0 to ±100 N in steps of 10 N.
4. Apply combined loading:
  1. permutations of internal external rotation moments of -5, 0, 5 Nm, varus-valgus moments of -10, 0, 10 Nm, and anterior-posterior drawer force of -100, 100 N.
5. Set flexion angle to 30°.
6. Apply laxity loading.
7. Apply combined loading.
8. Apply AP laxity loading at 30° flexion.
9. Set flexion angle to 60°.
10. Apply laxity loading.
11. Apply combined loading.
12. Set flexion angle to 90°.
13. Apply laxity loading.
14. Apply combined loading.
15. Apply AP laxity loading at 30° flexion.

Patellofemoral Joint Testing

Primary Conditions

Patellofemoral mechanics under quadriceps loading at tibiofemoral flexion angles of 0°, 15°, 30°, 45°, and 60°.

Secondary Conditions

Patellofemoral mechanics under quadriceps loading at flexion angles of 0°, 15°, 30°, 45°, and 60° with perturbed tibiofemoral internal-external rotation (+/- 10°)
Patellofemoral mechanics under quadriceps loading at flexion angles of 0°, 15°, 30°, 45°, and 60° with modified quadriceps line of action (+/- 5° varus/valgus).

Measurements

Joint kinematics
- joint kinematics are calculated based on Optotrak sensor output
- translations (anterior-posterior, compression-distraction, medial-lateral)
- rotations (flexion-extension, internal-external rotation, varus-valgus)
- provided in anatomical joint coordinate system
- provided in a human-readable or openly accessible form with clear descriptions of data structures and file naming/storage conventions
Contact pressures
- contact pressures are calculated based on Tekscan sensor outputs
- sensor location aligned with anatomical joint coordinate system
- provided in a human-readable or openly accessible form with clear descriptions of data structures and file naming/storage conventions

Equipment Use Modes

Refer to ["Infrastructure/ExperimentationMechanics"] for details of the equipment utilized in this specification. ["Specifications/SpecimenPreparation"] provides information on the state of the specimens before this testing starts.
Tibiofemoral joint loading will achieved using the six degrees of freedom robot (Rotopod R2000, PRSCO, Hampton, NH) with a rotary stage mounted to the top to yield a seventh degree of freedom for increased range of motion.
Quadriceps loading will be applied by a muscle actuator system.
- A linear actuator will be used for this purpose, see (["Infrastructure/ExperimentationMechanics"]).
- A freeze clamp will be used to attach the actuator to the tendon (["Specifications/SpecimenPreparation"]).
- The line of action will be setup to approximate the sulcus defined (inferior-superior) direction of the trochlear groove and will be placed accordingly before testing.
  - The line of action of the actuator will be quantified in a known coordinate frame during robot initialization, see ["Specifications/Registration"].
Joint kinematics will be measured using an Optotrak motion tracking system (NDI Corp., Ontario, Canada), which utilizes infrared emitting diode (IRED) markers:
- Triad marker clusters will be attached to each bone using the developed protocol.
- Kinematics will be described using a standard joint coordinate system, much like the tibiofemoral joint.
Pressure measurements will be accomplished using a pressure sensor (K-Scan sensor 5051, Tekscan Inc., MA) placed between the cartilage surfaces of the patella and femur
- Measures contact pressure distribution, area, and total force.
- Equivalencing and calibration will need to be performed before the testing, refer to ["Specifications/PressureCalibration"].
- Specimen preparation will be required to accommodate the sensor throughout the testing procedure. Details in ["Specifications/SpecimenPreparation"].

Operating Procedure

Testing of Primary Conditions:

The data (tibiofemoral and patellofemoral) will be collected and stored based on file naming and data storage conventions utilized by the BioRobotics Core of the Cleveland Clinic, see ["Infrastructure/ExperimentationMechanics"].
Tekscan pressure measurements output in the proprietary .fsx format. Pressure measurement file names will be specified as Specimen#_surgicalState_flexionAngle_quadLoad_Trial#.fsx.
- specimen# = specimen identifying number as provided by the cadaveric supplier. Specimen demographics will be provided for each specimen.
- surgicalState = This is the overall sate of the joint. This testing includes will be accomplished using a structurally intact joint. As such, it will be described as "intact." Details related to specimen preparation can be found in ["Specifications/SpecimenPreparation"]
- flexionAngle = tibiofemoral flexion angle
- quadLoad = specified quadriceps loading value
- trial = trial number. This should correspond to the trial number in the kinematics file as well.
- file naming conventions should allow for easy organizing of data with the robotic system data file structure conventions.
Assuming the tibiofemoral joint testing will be conducted prior to the patellofemoral joint testing, a readily available JCS will be used for tibiofemoral kinematics-kinetics control. If not, JCS optimization should be performed.
If desired, pre-conditioning of the patellofemoral joint should be performed by manually loading the knee. Our previous results indicated thet patellofemoral joint kinematics were insensitive to the load magnitude. Nonetheless, pre-conditioning may have an effect.
Before trials, ensure the pressure sensor is in place, see ["Specifications/SpecimenPreparation"].
The actions during the timeline of testing will be:
1. Set tibiofemoral joint kinematics
  - 0° flexion
  - Minimize out of plane loads (representation of passive flexion)
  - Lock tibiofemoral joint pose and orientation
2. Apply incremental quadriceps loads of ~20, 100, 200, 300, 400, 500, and 600 N
  - To accommodate the necessity for manual acquisition of patellofemoral pressure maps, quadriceps loading states will be manually controlled.
    - At 20 N quadriceps load, verify patellofemoral contact patch is approximately centered on the sensor. If not, center the sensor using the sutures (see ["Specifications/SpecimenPreparation"])
  - Once desired load is reached, data will be collected for approximately 5 seconds.
  - Record relative bone positions and patellofemoral contact pressures at each increment (follow file naming convention specified above).
  - Upon completion of quadriceps loading, specify zero force in the linear actuator and physically pull the quadriceps actuator to induce "slack" in the system (to facilite passive flexion)
3. Passively (minimize out of plane loads) flex the knee to a tibiofemoral joint angle of 15°
  - Lock the tibiofemoral joint pose and orientation
4. Apply incremental quadriceps loads and record bone positions and contact pressures (as at 0° flexion)
  - At 20 N quadriceps load, verify the pressure sensor is centered
  - Upon completion, induce slack in the tendon actuator
5. Passively flex the knee to a tibiofemoral joint angle of 30°
  - Lock the tibiofemoral joint pose and orientation
6. Apply incremental quadriceps loads and record bone positions and contact pressures (as at 0° flexion)
  - Verify the pressure sensor is centered
  - Upon completion, induce slack in the tendon actuator
7. Passively flex the knee to a tibiofemoral joint angle of 45°
  - Lock the tibiofemoral joint pose and orientation
8. Apply incremental quadriceps loads and record bone positions and contact pressures (as at 0° flexion)
  - Verify the pressure sensor is centered
  - Upon completion, induce slack in the tendon actuator
9. Passively flex the knee to a tibiofemoral joint angle of 60°
  - Lock the tibiofemoral joint pose and orientation
10. Apply incremental quadriceps loads and record bone positions and contact pressures (as at 0° flexion)
  - Verify the pressure center is centered
  - Verify pressure files are present and follow the above naming convention
  - Upon completion, induce slack in the tendon actuator and prepare to remove the specimen

References

Borotikar, Bhushan, Subject specific computational models of the knee to predict anterior cruciate ligament injury, Doctoral Dissertation, Cleveland State University, December 2009.

Grood ES, Suntay WJ. A joint coordinate system for the clinical description of three-dimensional motions: application to the knee. J Biomech Eng. 1983 May;105(2):136-44. [http://www.ncbi.nlm.nih.gov/pubmed/6865355 PubMed]